1. Field of the Invention
The present invention relates to an image pickup device and, in particular, to an image pickup device which uses an optical low pass filter of a phase type.
2. Description of the Related Art
In general, in a solid state image pickup element (such as a CCD) used in a video camera or an electronic still camera, an incident light image is sampled by sensors which are arranged in vertical and horizontal directions at given intervals from one another. For this reason, the highest spatial frequency that can be resolved is one-half of the sampling spatial frequency to be determined by the arrangement of the sensors, that is, a Nyquist frequency. If the incident light image includes spatial frequencies which are higher than the Nyquist frequency, the higher spatial frequencies will be returned in an ordinary band to produce a false signal or a moire effect.
Therefore, in the image pickup device of this type, such higher frequencies are removed by use of an optical low pass filter (which will be referred to as "OLPF" hereinafter) before sampling.
This OLPF includes a crystal OLPF using the double refraction of a crystal and a phase type OLPF having a surface varying unevenly at a given distance. The phase type OLPF is advantageous in that it can be produced in commercial quantity and thus it can be supplied at low costs. Normally, the OLPF is disposed nearer to the CCD side than to the magnification varying system of a taking lens.
Also, as an auto-focus device (AF device), there is used a device which performs a focusing detection and a focusing adjustment of the taking lens in accordance with an output signal from the CCD that photo-electrically converts an object light entering through the above OLPF. In other words, the AF device samples and integrates the high frequency component of the output signal obtained from the CCD, and by using the resultant integration value as an AF evaluation value for focusing decision, controls the focusing of the taking lens so that the AF evaluation value becomes the greatest.
However, if the phase type OLPF is disposed in front of a taking lens which has a variable focal distance (or magnification), then the phase type OLPF is not able to correspond to the variations of the focal distances of the taking lens and thus it is not able to remove a desired spatial frequency component.
On the other hand, if the phase type OLPF is disposed in front of the taking lens, then the back length of the taking lens can be reduced to thereby provide an advantage in space and to decrease the possibility that a ghost image can be produced. Further, in this case, it is possible to increase the given distance between the surface unevennesses of the phase type OLPF to thereby be able to provide an easy optical pattern. Also, in the back focus adjustment of the taking lens, the OLPF can be disconnected from the optical system with ease so that the back focus of the taking lens can be adjusted easily.
However, in the AF device of the above-mentioned conventional image pickup device, as shown by a solid line in FIG. 6, due to the fact that the focusing detection is executed by the OLPF in accordance with a video signal (that is, the information contained in a range A shown by oblique lines) in which a modulation transfer function of a given spatial frequency (f.sub.c) has been lowered, the amount of information is decreased and it is difficult to focus an object which has the above-mentioned given spatial frequency component.